The present invention relates to a light guide plate and a planar lighting device that may be used in liquid crystal displays and the like.
A liquid crystal display uses a planar lighting device (a backlight unit) which illuminates a liquid crystal display panel by irradiation of light from the back side of the liquid crystal display panel. The backlight unit is configured using a light guide plate for diffusing light emitted from an illumination light source to illuminate the liquid crystal display panel and parts such as a prism sheet and a diffusion sheet for making outgoing light from the light guide plate uniform.
Currently, large-sized liquid crystal televisions predominantly use a so-called underneath type backlight unit including a light guide plate disposed immediately above an illumination light source. This type of backlight unit ensures uniform light amount distribution and necessary luminance by disposing a plurality of cold cathode tubes used as light sources behind the liquid crystal display panel and having white reflection surfaces inside the backlight unit.
However, the underneath type backlight unit requires a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel in order to make the light amount distribution uniform and further reduction in thickness is difficult to achieve.
On the other hand, an exemplary backlight unit that allows the thickness reduction includes one using a light guide plate which receives light emitted from an illumination light source, guides the received light in predetermined directions and emits the guided light through a light exit surface that is different from the surface through which the light entered.
As the backlight unit using the light guide plate as described above, there has been proposed a backlight unit of a type using a light guide plate in plate form which receives light through its lateral faces and emits the received light through the front face by incorporating scattering particles for scattering light in a transparent resin.
For example, JP 7-36037 A describes a light scattering and guiding light source device comprising a light scattering guide having at least one light incidence surface region and at least one light exit surface region and light source means for light incidence through the light incidence surface region, the light scattering guide having a region that has a tendency to decrease in thickness with increasing distance from the light incidence surface.
JP 8-248233 A describes a surface light source device comprising a light scattering guide, a prism sheet provided on the side of the light scattering guide closer to the light exit surface, and a reflector provided on the back side of the light scattering guide. JP 8-271739 A describes a liquid crystal display comprising a light emitting direction correcting element which is formed of a plate-shaped optical material and includes a light incidence surface having a repeated undulate pattern of prism arrays and a light exit surface having light diffusing properties. JP 11-153963 A describes a light source device comprising a light scattering guide provided with scattering ability inside and light supply means for supplying light through an end face portion of the light scattering guide.
Also proposed in addition to the above light guide plates are a light guide plate having a larger thickness at the center thereof than at an end on the light incidence side and at the opposite side end; a light guide plate having a reflection surface inclined in such a direction that the thickness of the light guide plate increases with increasing distance from a light entrance portion; and a light guide plate having such a shape that the distance between the front surface portion and the rear surface portion is the smallest at the light entrance portion and that the thickness of the light guide plate is the largest when the distance from the light entrance portion is the largest (see, for example, JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, JP 2005-302322 A and JP 8-220346 A).
In addition, JP 2009-117349 A describes a lighting device having a light guide plate of which the light exit surface is concave, and JP 2009-117357 A describes a light guide plate of which the light exit surface is downwardly convex (i.e., the light exit surface is concave).
JP 2009-117357 A discloses a two-layer light guide plate in which the interface between the first layer and the second layer is inclined in the directions approaching the light exit surface with increasing distance from the ends toward the center of the light guide plate (so that the cross section is in the shape of an isosceles triangle).
In addition, JP 4127897 B (JP 11-345512 A) describes a planar light source device comprising a sheet member in which at least one non-scattering light guide region and at least one scattering light guide region containing particles with a different refractive index uniformly dispersed in the same material as the non-scattering light guide region overlap each other, a light source lamp being mounted on one end face, the distribution state of the amount of light emitted from the main surface being controlled by locally adjusting the particle concentration in the sheet thickness of both the regions, the scattering light guide region including convex light guide blocks and the non-scattering light guide region including concave light guide blocks corresponding to the convex light guide blocks.